WHERE IS THE CHEMOCLINE? PART 5: WHY THE ANCIENT CHEMOCLINE MATTERS TODAY
Welcome to the fifth and final segment in this blog series on the chemocline. We have covered a lot of ground from discussions on pyrite framboids HERE, to pyrite fossil beds HERE, and geochemistry HERE. I would like to wrap up this series by discussing why I think understanding these ideas is important.
I have spent the better part of the last 20 years researching black shales. Much of this research focused on unconventional hydrocarbon production from organic-rich shale and mudstone. That organic-rich rocks were the sources of petroleum and often yielded hydrocarbon shows while drilling was no real secret. Indeed, the U.S. Geological Survey released “Petroleum Geology of the Devonian and Mississippian Black Shale of Eastern North America” in 1993.
The loose connection between hydrocarbon accumulation and black shale deposits was beginning to form. For my part, I started to look at horizontal wellbore placement or “targeting” shortly after entering the industry in 2006. I began to see the link between individual well performance and stratigraphic position of the wellbore in the Marcellus Shale. It wasn’t long before I realized that wells placed in the highest TOC rock had the best performance.
However, it wasn’t until 2009 when Loucks et al. published the first SEM images from Ar-ion milled samples that we realized that organic matter was the primary host of porosity in shales. The Marcellus Shale is no exception:
Indeed, after exhaustive research by many workers over the past decade, organic-matter hosted pores seem to play a role, if not the primary role in pore systems of all organic-rich mudstone plays. All across the Marcellus depositional trend, regardless of sedimentology, thermal maturity, or structural deformation, a strong correlation exists between porosity and organic matter:
Depositional conditions play a fundamental role in the production, dilution, and preservation of organic matter. High productivity waters with high clastic influx yield low organic-content mudstones due to dilution. Moderate to even low productivity waters may produce high TOC shales if clastic influx is at a minimum. Moreover, the carbon-oxygen-sulfur ratios of organic matter play a large role in thermal stress needed to break organic matter down into hydrocarbon. For example, C-S bonds require lower activation energies than do C-O or C-C bonds. Thus, as thermal maturity increases, sulfurized organic matter (which may be common when organic matter is deposited under euxinic conditions) will begin to break down and generate hydrocarbon before non-sulfurized organic matter.
Finally, it is worth mentioning that black shale offers more than just hydrocarbon to the economy. The deposits are likely the largest accumulation of critical minerals (including rare earth elements) on the planet. The hurdle to extracting these elements has always been the cost associated with processing high volume/low concentration deposits. The concentration and distribution of these critical minerals is strongly influenced by depositional environment, where deposition under anoxic conditions tends to lead to the accumulation of many critical minerals of interest.
If I could sum it all up in a brief statement, I would say that Appalachian Basin black shales deposited under anoxic to euxinic conditions positively impact the accumulation and preservation of both organic matter and critical minerals. Understanding the stratigraphic and spatial distribution of these components requires understand the depositional environments, bottom water conditions, and basin hydrography across space and time. One of the many aspects that influence these parameters is the position of the chemocline, and thus tracking its position is of the utmost importance.